Fix python detection

[BusyJay]

Some platform, like ArchLinux, may not have python in the PATH, but can still have python2. So we should add some fallback option to detect python correctly. This detection order is copy from rust-lang/rust.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[alexcrichton]

@bors: r+ f97f854

Thanks!

[bors]

⌛ Testing commit f97f854 with merge 5e2a949...

[alexcrichton]

@bors: retry force clean

[bors]

⌛ Testing commit f97f854 with merge 6440616...

[bors]

💔 Test failed - cargo-linux-32

[alexcrichton]

@bors: retry

On Mon, Jul 25, 2016 at 11:48 AM, bors notifications@github.com wrote:

💔 Test failed - cargo-linux-32
https://buildbot.rust-lang.org/builders/cargo-linux-32/builds/1736

—
You are receiving this because you were mentioned.
Reply to this email directly, view it on GitHub
#2910 (comment), or mute
the thread
https://github.com/notifications/unsubscribe-auth/AAD95MJFzhsRCqdtygIO8WYlY9TElHC9ks5qZQUagaJpZM4JTcZ2
.

[bors]

⌛ Testing commit f97f854 with merge c547d97...

[bors]

☀️ Test successful - cargo-cross-linux, cargo-linux-32, cargo-linux-64, cargo-mac-32, cargo-mac-64, cargo-win-gnu-32, cargo-win-gnu-64, cargo-win-msvc-32, cargo-win-msvc-64
Approved by: alexcrichton
Pushing c547d97 to master...